Materials that have been conventionally used for sliding members include compositions in which solid lubricant materials such as PTFE or graphite and fibrous reinforcing materials such as carbon fibers or glass fibers are added to synthetic resins such as polyimides (hereunder, “PI”), polyether ether ketones (hereunder, “PEEK”), polyamideimides (hereunder, “PAI”) and polyphenylene sulfides (hereunder, “PPS”). In such resin compositions, addition of a solid lubricant material lowers the frictional coefficient while addition of a fibrous reinforcing material increases the wear resistance, the mechanical strength and the creep resistance, so that a material with excellent sliding properties and mechanical properties is obtained. In recent years, however, with increasing market demands for higher energy efficiency and lower fuel consumption, there is a strong need for reduction in drag torque (the frictional coefficient μ) on sliding parts such as seal rings, in particular. Even when a large amount of a fluorine-based resin such as PTFE is added in order to further improve the sliding properties, a sufficient lubricating effect is not obtained and the mechanical strength, including high-temperature rigidity, is reduced. On the other hand, when the fibrous reinforcing material is increased to further improve the mechanical strength, problems occur such as damage to counterpart materials. Therefore, numerous types of resin compositions have been proposed with the aim of improving the sliding properties and mechanical properties.
For example, Patent Literature 1 proposes a sliding member composition comprising 0.25 to 10 vol % of one or more types of inorganic oxide fine particles with a Moh's hardness (former) of 2.5 to 7 and a particle size of 0.1 to 30 μm; 3 to 30 vol % of a solid lubricant material; and the remainder of a synthetic resin. Such a composition also containing an aromatic polyamide fiber added to 0 to 30 vol % is also mentioned. In Patent Literature 1, it is stated that excellent wear resistance and a low frictional coefficient are obtained by filling with a specified amount of inorganic oxide fine particles with a hardness in a specified range and particle size in a specified range, in addition to a solid lubricant material.
Also, Patent Literature 2 discloses a polyether ketone-based resin composition with addition of 10 to 90 parts by weight of a copolymer of 3,3,3-trifluoro-2-trifluoromethylpropene and 1,1-difluoroethylene with respect to 100 parts by weight of a polyether ketone resin. It is stated that the composition of Patent Literature 2 has excellent non-adhesiveness, without impairment of the inherent mechanical, thermal and electrical properties of the polyether ketone resin, and also exhibits very desirable sliding properties, and is thus optimal as a sliding member to be used at high temperatures.
However, with the resin composition for a sliding member according to Patent Literature 1, fine particles of an inorganic oxide such as a metal oxide are an essential component, and therefore the extent of wear of counterpart materials can potentially increase. Furthermore, addition of a high density metal oxide generally increases the specific gravity, impairing the lightweight advantage of the resin material.
On the other hand, addition of a low-molecular-weight fluorine-containing compound such as in Patent Literature 2 may be expected to improve dispersibility and enhance the physical properties of the material. However, using such a low-molecular-weight fluorine-containing compound can result in phase separation after mixing and lower heat resistance, and under harsh conditions it may not be possible to exhibit sufficient sliding properties.